B Porter - Carpentry and Joinery-B-H (2001)

Prévia do material em texto

Carpentry and Joinery 1
This Page Intentionally Left Blank
Carpentry and Joinery
Third Edition
1
Brian Porter LCG, FIOC
Formerly of Leeds College of Building
First published in Great Britain 1984
Second edition 1991 by Edward Arnold
Third edition 2001 by Butterworth Heinemann
© 2001 Brian Porter
All rights reserved. No part of this publication may be reproduced or
transmitted in any from or by any means, electronically or mechanically,
including photocopying, recording or any information storage or
retrieval system, without either prior permission in writing from the
publisher or a licence permitting restricted copying. In the United
Kingdom such licences are issued by the Copyright Licensing Agency:
90 Tottenham Court Road, London W1P 9HE.
Whilst the advice and information in this book is believed to be true
and accurate at the date of going to press, neither the author[s] nor the
publisher can accept any legal responsibility or liability for any errors or
omissions that may be made.
British Library Cataloguing in Publication Data
Porter, Brain 1938–
Carpentry and joinery. – 3rd ed.
Vol. 1
1. Carpentry and Joinery.
I. Title
694
ISBN 0 000 00000 0
Design and Typesetting by J&L Composition Ltd, Filey, North Yorkshire
Printed and bound in Great Britain by
Foreword ix
Preface to the First Edition x
Preface to the Second Edition x
Preface to the Third Edition xi
Acknowledgements xii
CHAPTER ONE TIMBER 1
1.1 Growth and structure or a tree 1
1.2 Hardwood and softwood trees 4
1.3 Forest distribution (Source and supply of timber) 5
1.4 Conversion into timber 9
1.5 Size and selection of sawn timber 14
1.6 Structural defects (natural defects) 15
1.7 Drying timber 18
1.8 Grading timber 31
1.9 Processing squared sectioned timber 39
1.10 Structure of wood and identification of timber 44
1.11 Properties of timber 55
CHAPTER TWO ENEMIES OF WOOD AND WOOD BASED PRODUCTS 60
2.1 Non-rotting fungi (Sap-staining fungi) 61
2.2 Wood-rotting fungi 61
2.3 Attack by wood boring insects 66
CHAPTER THREE WOOD PRESERVATION AND PROTECTION 74
3.1 Paint and varnishes 74
3.2 Water-repellent exterior stains 75
3.3 Preservatives 75
3.4 Methods applying preservatives 78
3.5 Flame-retardant treatments 81
3.6 Other treatments 81
3.7 Health and safety 81
CHAPTER FOUR MANUFACTURED BOARDS AND PANEL PRODUCTS 84
4.1 Veneer plywood 84
4.2 Core plywood 89
4.3 Chipboard 90
4.4 Wood-cement particleboard 93
Contents
4.5 Oriented Strand Board OSB 94
4.6 Fibre building boards 96
4.7 Laminated plastics (Decorative Laminates) 101
4.8 Fibre cement building boards 105
4.9 Plasterboards 106
4.10 Composite boards 107
4.11 Conditioning wood-based boards and other sheet materials 107
4.12 Storage and stacking 108
4.13 Handling 109
4.14 Health & safety 109
CHAPTER FIVE HANDTOOLS AND WORKSHOP PROCEDURES 111
5.1 Measuring tools 111
5.2 Setting-out, marking-out & marking-off tools 112
5.3 Saws 117
5.4 Planes 122
5.5 Boring tools 128
5.6 Chisels (wood) 135
5.7 Shaping tools 137
5.8 Driving (Impelling) tools 138
5.9 Lever & withdrawing tools 146
5.10 Finishing tools & abrasives 147
5.11 Holding equipment (tools & devices) 149
5.12 Tool storage & accessory containers 153
5.13 Tool maintenance 159
CHAPTER SIX PORTABLE ELECTRIC MAINS POWERED HAND TOOLS & MACHINES 167
6.1 Specification plate (SP) 167
6.2 Earthing, insulation & electrical safety 168
6.3 Use of portable power tools 171
6.4 Electric drills (rotary) 171
6.5 Rotary impact (percussion) drills 174
6.6 Rotary hammer drills 175
6.7 Drill chucks 175
6.8 Electric screwdrivers 176
6.9 Sanders 177
6.10 Circular saws 178
6.11 Mitre saws 181
6.12 Combination saw bench and Mitre saws 183
6.13 Reciprocating saws 183
6.14 Planers 186
6.15 Routers 187
6.16 Nail and staple guns 191
CHAPTER SEVEN BATTERY-OPERATED (CORDLESS) HAND TOOLS 193
7.1 Method of use 193
7.2 Safe operation 193
vi Contents
CHAPTER EIGHT CARTRIDGE OPERATED FIXING TOOLS (BALLISTIC TOOLS) 196
8.1 Types of tool 196
8.2 Cartridges 197
8.3 Fixing devices 198
8.4 Base materials 199
8.5 Fixing to concrete 201
8.6 Fixing into steel (usually structural mild steel sections) 202
8.7 Safe operation 202
CHAPTER NINE BASIC STATIC WOODWORKING MACHINES 205
9.1 Crosscutting machines 206
9.2 Hand feed circular saw benches 209
9.3 Dimension saw 211
9.4 Panel saws 212
9.5 Saw blades 213
9.6 Planing machines 216
9.7 Narrow bandsaw machines 222
9.8 Mortising machines 227
9.9 Sanding machines 229
9.10 Grinding machines 230
9.11 Workshop layout 232
9.12 Safety 232
CHAPTER TEN BASIC WOODWORKING JOINTS 235
10.1 Lengthening – end joints 235
10.2 Widening – edge joints 235
10.3 Framing – angle joints 238
CHAPTER ELEVEN WOOD ADHESIVES 245
11.1 Adhesive types 245
11.2 Adhesive characteristics 247
11.3 Application of adhesives 248
11.4 Safety precautions 248
CHAPTER TWELVE FIXING DEVICES 249
12.1 Nails 249
12.2 Wood screws 251
12.3 Threaded bolts 255
12.4 Fixing plates 255
12.5 Plugs 257
12.6 Combination plugs 258
12.7 Cavity fixings 261
12.8 Anchor bolts 262
Contents vii
PRACTICAL PROJECTS 268
1: Porterbox – drop fronted tool box and saw stool 268
2: Portercaddy 272
3: Portercase 275
4: Porterdolly 276
5: Porterchest 277
6: Porterbench 282
7: Portertrestle – traditional saw stool 286
INDEX 293
viii Contents
The craft of the carpenter and joiner, at least in
those areas of the world where there is plentiful
supply of timber is as old as history, and this
book describes and illustrates for the benefit of
students and others who care to read it, the
changing techniques that continue to take place
as our knowledge of wood and its working devel-
ops. Standards controlling the quality of timber,
timber based products, workmanship and safe
working practices are continually being revised
and harmonised to meet, not only our higher
standards, but also those of Europe. Improved
fastenings and adhesives have revolutionised
joining techniques. Development in electrical
battery technology has made possible the cord-
less power tool. These improvements in the field
of woodworking are a continual process, and so
must be the updating of textbooks to reflect
these changes. Brian Porter as a practicing
Carpenter and Joiner, and a lecturer in wood
trades is familiar with these changes, which have
been incorporated into this revised edition, writ-
ten to help wood trade students in the early
stages of their chosen careers understand the
techniques and principles involved in the safe
and efficient working of timber and timber prod-
ucts. A book maintaining the high standard Brian
Porter set himself in his earlier publications, and
which provides a wealth of information that will
be helpful to all who have an interest in the
working of wood.
Reg Rose MCIOB, DMS, DASTE,
FIOC former Assistant Principal,
Leeds College of Building, UK
Foreword
I find it difficult to comprehend that after 20
years and two previous editions, this book is still
in demand across the world. Its content has of
course been periodically updated in keeping
with current trends and legislation, but in
essence, it remains the same.
This new edition is printed in a similar format
to the last one, but as can be seen from the con-
tent page similarities end there. Its new design
has taken into account the necessary theoretical
job knowledge requirements of the modern car-
penter and joiner. But, as with all my previous
books, still maintaining a highly visual approach
to its content.
It would appear to some people that in recent
years, many of our traditional hand tool skills
have been replaced by the used of portable
power tools. To some extent this may be partly
true, but I believe that in the majority of cases,
the professional carpenter and joiner would
regard portable power tools more as an aid to
greater productivity, rather than a replacement
for traditional hand tools.
Hand tools still play a vital part in our work,
and it is for this reason that I haveagain included
a large section devoted to their selection, safe use
and application – together with several pieces of
ancilliary equipment.
Carpentry and Joinery volume 1, can also be
used by students to help them grasp basic under-
pinning knowledge of many, if not all of our
every day work activities. It should be particulary
helpful as a basis for acquiring a greater under-
standing of the activities set out in the new
editions of Carpentry and Joinery volumes 2
and 3.
Unlike previous editions, the practical projects
now appear within their own section towards the
back of the book. To accompany the now well
established ‘Porterbox’ system of containers, two
additional projects have been added to this sec-
tion – a ‘saw stool’ and ‘workbench’.
I feel confident that the readers of this book
will find it not only an asset to gaining a greater
understanding of our craft, but also as a refer-
ence manual for future use.
Brian Porter 2001
Preface to the second edition
The main difference between this book – the
first of three new editions – is the overall size and
format of the contents compared to the previous
first edition (published in 1982). The most
important reason for this change is that in the
majority of cases text and illustration can now
share the same or adjacent page, making refer-
ence simpler and the book easier to follow. The
most significant change of all is the new section
on tool storage: Several practical innovative
projects have been included which will allow
the reader to make, either as part of his or
her coursework or as a separate exercise, a sim-
ple, yet practical system of tool storage units
and tool holders – aptly called the ‘Porterbox’
system (original designs were first published in
Woodworker magazine). Each chapter has been
reviewed and revised to suit current changes. For
example, this has meant the introduction of new
hand tools, replacing or supplementing existing
portable powered hand tools, and updating some
woodworking machines.
Educational and training establishments seem
to be in a constant state of change; college and
school based carpentry and joinery courses are
no exception. As the time available for formal
tuition becomes less, course content, possibly
due to demands made by industry and the intro-
duction of new materials together with a knowl-
edge of any associated modern technology they
bring with them, seems to be getting greater –
making demands for support resource material
probably greater than they have ever been.
Distance learning (home study with profes-
Preface
sional support) can have a very important role to
play in the learning process, and it should be
pointed out that in some areas of study it is not
just an alternative to the more formally struc-
tured learning process, but a proven method in
its own right.
No matter which study method is chosen by
the reader, the type of reading matter used to
accompany studies should be easy to read and
highly illustrative, and all subjects portrayed
throughout this book meet that requirement. I
hope therefore that this book is as well read, and
used, by students of this most fulfilling of crafts.
Brian Porter
Leeds 1989
Preface to the first edition
This volume is the first of three designed to meet
the needs of students engaged on a course of
study in carpentry and joinery. Together, the
three volumes cover the content of the City
and Guilds of London Institute craft certificate
course in carpentry and joinery (course number
585).
I have adopted a predominantly pictoral
approach to the subject matter and have tried to
integrate the discussion of craft theory and asso-
ciated subjects such as geometry and mensura-
tion so that their interdependence is apparent
throughout. However, I have not attempted to
offer instruction in sketching, drawing, and per-
spective techniques (BS 1192), which I think are
best left to the individual student’s school or
college.
Procedures described in the practical sections
of the text have been chosen because they follow
safe working principles – this is not to say that
there are no suitable alternatives, simply that I
favour the ones chosen.
Finally, although the main aim of the book is
to supplement school or college-based work of a
theoretical and practical nature, its presentation
is such that it should also prove invaluable to
students studying by correspondence course
(‘distance learning’) and to mature students who
in earlier years may perhaps have overlooked the
all-important basic principles of our craft.
Brian Porter
Leeds 1982
Contents xi
I wish to thank:
Reg Rose for proof reading the text, writing the foreword, and allowing me to reproduce many pieces
of artwork we shared in previous joint authorship work as listed on the back of this book.
Eric Cannell for editing and contributing material for Chapter 9 (Woodworking Machines).
Peter Kershaw (Managing Director North Yorkshire Timber Co Ltd.) for his help and guidance.
Colleagues and library staff at Leeds College of Building.
I would also like to offer my gratitude to the following people and companies for their help and support
by providing me with technical information and in many cases permission to reproduce item of artwork
and/or photographs.Without their help many aspects of this work would not have been possible.
Arch Timber Protection (formerly ‘Hickson Timber Products Ltd.)
A L Daltons Ltd (Woodworking machines)
American Plywood Association (APA)
Black & Decker
British Gypsum Ltd.
Cape Boards Ltd.
Cape Calsil Systems Ltd.
Council of Forest Industries (COFI)
CSC Forest Products (Sterling) Ltd.
Denford Machine Tools Co Ltd.
DEWALT
Draper (The Tool Company).
English Abrasive & Chemicals Ltd.
Fibre Building Board Organisation
Finnish Plywood International
Fischer Fixing Systems.
Forestor – Forest and Sawmill Equipment (Engineers) Ltd.
Formica Ltd.
Fosroc Ltd.
G. F. Wells Ltd. (Timber Drying Engineers).
Hilti Ltd.
ITW Construction Products (Paslode)
Kiln Services Ltd.
Louisiana-Pacific Europe
Makita UK Ltd.
Mr Stewart J. Kennmar-Glenhill (Imperial College of Science and Technology, London) and David
Kerr and Barrie Juniper of the Plant Science Department, Oxford, for contributing Figures 1.3 to
1.6
Mr. John Common (Kiln Services Ltd.)
Neil Tools Ltd.
Acknowledgements
Nettlefolds (Woodscrews)
Nordic Timber Council
North Yorkshire Timber Co Ltd.
Perstorp Warerite Ltd.
Protim Ltd.
Protimeter PLC
Rabone Chesterman Ltd.
Record Marples (Woodworking Tools) Ltd.
Record Tools Ltd.
Rentokil Ltd.
Robert Bosch Ltd. (Power Tools Division)
Stanley Tools, Stanley Works Ltd.
Stenner of Tiverton Ltd.
The Rawlplug Company Ltd.
Timber Research and Development Association (TRADA), for information gleaned from TRADA
Wood Information Sheets’.
Trend Machinery & Cutting Tools Ltd.
Wadkin Group of Companies PLC.
Willamette Europe Ltd.
Wolfcraft
Tables 1.2 and 1.4 are extracted from BS 4471
Table 1.3 is extracted from BS 5450
Copies of the complete standards can be obtained from British Standards Institute, 389
Chiswick High Road, London W4 4AL. Copyright is held by the Crown and reproduced with kind
permission of the British Standards Institute.
With kind permission the line drawings in figures 9.1 (WIS 31). 9.3 (WIS 31). 9.7 (WIS 16). 9.25
(WIS 17). 9.26 (WIS 17). 9.27 (WIS 17). 9.30 (WIS 17). and 9.42 (WIS 31). were extracted from
‘Health & Safety Executive’ (HSE) Woodworking Information Sheets (WIS).
The full information sheets are available from:
HSE Books
PO Box 1999
Sudbury
Suffolk CO10 2WA
Acknowledgements xiii
This Page Intentionally Left Blank
As part of their craft expertise, carpenters and
joiners should be able to identify common, com-
mercially used timbers and manufactured
boards, to the extent that they also become
aware of how they (as shown in figure 1.1) will
respond to being:
a cut by hand and machine,
b bent,
c subjected to loads,
d nailed and screwed into,
e glued,
f subjected to moisture,
g attacked by fungi,
h attacked by insects,
i subjected to fire,
j treatedwith preservatives, flame retardants,
sealants, etc.,
k in contact with metal.
By and large, behaviour under these conditions
will depend on the structural properties of the
timber, its working qualities, strength and resist-
ance to fungal decay (durability), insect attack,
chemical make-up, and moisture content.
1.1 Growth & structure of a tree
The life of a tree begins very much like that of
any other plant – the difference being that, if
the seedling survives its early stage of growth
to become a sapling (young tree), it may dev-
elop into one of the largest plants in the plant
kingdom.
The hazards to young trees are many and var-
ied. Animals are responsible for the destruction
of many young saplings, but this is often regar-
ded as a natural thinning-out of an otherwise
overcrowded forest, thus, allowing the sapling to
mature and develop into a tree of natural size
and shape. Where thinning has not taken place,
trees grow thin and spindly – evidence of this
can be seen in any overgrown woodland where
trees have had to compete for the daylight nec-
essary for their food production.
With all natural resources that are in constant
demand, there comes a time when demand
outweighs supply. Fortunately, although trees
require 30–100 years or more to mature, it is
possible to ensure a continuing supply – prov-
ided that land is made available and felling, (cut-
ting down) is strictly controlled. This has meant
that varying degrees of conservation have had to
be enforced throughout some of the world’s
largest natural forests and has led to the devel-
opment of massive man-made forests (forest
farming).
Timber
1
Fig 1.1 Timber may respond differently to these
treatments
(a)
(b)
(c)
(e)
(d)
(f)
(g)
(h)
(k)
(j)
(i)(i)
1.1.1 Tree components
There are three main parts:
● the root system,
● the stem or trunk,
● the crown.
As can be seen from figure 1.2 these can vary
with the type of tree (section 1.2).
a Root system – The roots anchor the tree
firmly into the ground, these can exceed the
radius of the tree – size and spread depends
on type and size of tree. The many small root
hairs surrounding the root ends, absorb water
and minerals, to form sap (see fig. 1.3).
b Trunk (stem) – The stem or trunk conducts
sap from the roots, stores food, and supports
the crown. When the trunk is cut into lengths
– they are called logs or boules. Timber is cut
from this part of the tree (see fig. 1.3).
c Crown – The crown consists of branches,
twigs, and foliage (leaves). Branches and
twigs are the lifelines supplying the leaves
with sap (see fig. 1.3).
1.1.2 The food process (fig. 1.4)
The leaves play the vital role of producing the
tree’s food. By absorbing daylight energy via the
green pigment (chlorophyll) in the leaf, they
convert a mixture of carbon dioxide taken from
both the air and sap from the roots into the nec-
essary amounts of sugars and starches (referred
to as food) while, at the same time releasing oxy-
gen into the atmosphere as a waste product.This
process is known as photosynthesis. However,
during the hours of darkness, this action to some
extent is reversed – the leaves take in oxygen and
give off carbon dioxide, a process known as
respiration (breathing).
For the whole process to function, there must
be some form of built-in system of circulation
which allows sap to rise from the ground to the
leaves and then to descend as food to be distrib-
uted throughout the whole tree. It would seem
that this action is due either to suction, induced
by transpiration (leaves giving off moisture by
evaporation), and/or to capillarity (a natural ten-
dency for a liquid to rise within the confines of
the cells – see Table 1.12) within the cell struc-
ture of the wood.
1.1.3 Structural elements of a tree
We will start from the outer circumference of the
tree and work towards its centre. The following
features are illustrated in fig. 1.5.
a Bark – the outer sheath of the tree. It
functions as:
● a moisture barrier,
● a thermal insulator, against extremes in
temperature – both hot or cold.
● an armour plate against extremes of
temperature, attack by insects, fungi, and
animals.
The bark of a well-established tree can
usually withstand minor damage, although
excessive ill treatment to this region could
prove fatal.
b Inner bark (bast or phloem) – Conducts
food throughout the whole of the tree, from
the leaves to the roots.
c Cambium (fig. 1.6) – A thin layer or sleeve
of cells located between the sapwood and the
bast (phloem). These cells are responsible for
the tree’s growth. As they are formed, they
become subdivided in such a way that new
cells are added to both sapwood and
phloem, thus increasing the girth of the tree.
d Growth ring – (Sometimes referred to as an
annual ring) – wood cells that have formed
around the circumference of the tree during
its growing season. The climate and time of
2 Timber
Crown
Root system
(often shallow)
Narrow (needle)
leaved (conifer) tree
Root system
(usually deep)
Broadleaved tree
Crown
Stem
(Trunk)
Fig 1.2 General tree shape with their main parts
year dictate the growth pattern. Each ring is
often seen as two distinct bands, known as
earlywood (springwood) and latewood
(summerwood). Latewood is usually more
dense than earlywood and can be recognised
by its darker appearance.
Growth rings are important because they
enable the woodworker to decide on the
suitability of the wood as a whole – either as
timber for joinery (appearance & stability)
or, its structural properties (strength &
stability) as carcaseing timber.
Growth & structure of a tree 3
HARDWOOD SOFTWOOD
Foliage
Branch
Trunk
Food
Sap
First grown
Conduction
Water and minerals
Root system
Fig 1.3 Growth of a tree (hardwood & softwood)
e Rays – these may all appear (although
falsely – as not many do) to originate from
the centre (medulla) of the tree, hence the
term medullary rays is often used to describe
this strip of cells that allow sap to percolate
transversely through the wood. They are also
used to store excess food.
Rays are more noticeable in hardwood
than in softwood (see figure 1.83), and even
then can be seen with the naked eye only in
such woods as oak and beech. fig. 1.21
shows how rays may be used as a decorative
feature once the wood has been converted
(sawn into timber).
f Pith (medulla) – the core or centre of the
tree, formed from the tree’s earliest growth
as a sapling. Wood immediately surrounding
the pith is called juvenile wood, which is not
suitable as timber.
g Sapwood – the outer active part of the tree
which, as its name implies, receives and
conducts sap from the roots to the leaves. As
this part of the tree matures, it gradually
becomes heartwood.
h Heartwood – the natural non-active part of
the tree, often darker in colour than
sapwood, gives strength and support to the
tree and provides the most durable wood for
conversion into timber.
1.2 Hardwood & softwood trees
The terms hardwood and softwood can be very
confusing, as not all commercially classified
hardwoods are physically hard, or softwoods
soft. For example, the obeche tree is classed as a
hardwood tree, yet it offers little resistance to a
saw or chisel etc. The yew tree, on the other
hand, is much harder to work yet it is classified
as a softwood.To add to this confusion we could
be led to believe that hardwood trees are decid-
uous (shed their leaves at the end of their grow-
ing season) and softwood trees are evergreen
(retain their leaves for more than one year),
4 Timber
Light energy
ChlorophyII
Sa
p
Oxygen
Oxygen
Carbon
dioxide
Carbon
dioxide Fo
o
d
Fo
o
d
S
ap
Daylight Darkness
Fig 1.4 The process of photosynthesis
(e) Rays
(b) Inner bark or bast (phloem)
(a) Bark
(c) Cambium
(f) Pith (medulla)
(h) Heartwood
(g) Sapwood
Note: Sectional details of
softwood (d)
Growth ring
(annual ring)
Latewood (Summerwood)
Earlywood (Springwood)
Fig 1.5 Section through the stem/trunk
which is true of most species within these
groups, but not all!Table 1.1 identifies certain characteristics
found in hardwood and softwood trees; however,
it should be used only as a general guide.
Hardwood and softwood in fact, refer to
botanical differences in cell composition and
structure (Cell types and their formation are
dealt with in section 1.10).
1.2.1 Tree & timber names
Common names are often given to trees, (and
other plants) so as to include a group of similar
yet botanically different species. It is these com-
mon English names which are predominantly
used in our timber industry. The true name or
Latin botanical name of the tree must be used
where formal identification is required – for
example:
Species (true name or
Latin botanical name)
Genus
Common name (generic name Specific name
English name or ‘surname’) (or ‘forename’)
Scots pine Pinus sylvestris
As a general guide, it could therefore be said that
plants have both a surname and a forename,
and, to take it a step further, belong to family
groups of hardwood and softwood.
Commercial names for timber often cover
more than one species. In these cases, the botan-
ical grouping is indicated by ‘spp’, telling us that
similar species may be harvested and sold under
one genus (singular of genera).
Table 1.2 should give you a better idea how
family groups are formed. The first division is
into hardwoods and softwoods, next, into their
family group, this is followed by their ‘Genera’
group, then finally their species.
1.3 Forest distribution (source and
supply of timber)
The forests of the world that supply the wood for
timber, veneers, wood pulp, and chippings for par-
ticle board, are usually situated in areas which are
typical for a particular group of tree species. For
example, as can be seen from figure 1.7, the conif-
erous forests supplying the bulk of the world’s soft-
woods are mainly found in the cooler regions of
northern Europe, also Canada and Asia – stretch-
ing to the edge of the Arctic Circle. Hardwoods,
however, come either from a temperate climate
(neither very hot nor very cold) – where they are
mixed with faster-growing softwoods – or from
subtropical and tropical regions, where a vast
variety of hardwoods grow.
There is increasing concern about the envi-
ronmental issues concerning forest manage-
ment, particularly with regard to over extraction
of certain wood species – many of which are
now protected. This concern has led to some
suppliers certifying that their timber is from a
Forest distribution (source and supply of timber) 5
Conduction of water
 and sap to foliage
H
e
ar
tw
o
o
d
sa
p
w
o
o
d
Xy
le
m
 (
w
o
o
d
)
D
iv
id
es
 to
 p
ro
du
ce
 n
ew
 c
el
ls
 to
 fo
rm
 p
hl
oe
m
 &
 x
yl
em
C
am
b
iu
m
In
n
er
 b
ar
k 
o
r 
b
as
t 
(p
h
lo
em
)
O
u
te
r 
b
ar
k
Note:
Phloem - pronounced 'flo-em'
Xylem - pronounced 'zi-lem'
Fig 1.6 Function of the cambium layer
non-protected or sustainable source. Suppliers
are now being urged to specify that any timber or
wood product used, should be able to present a
copy of their Environmental Policy with regard
to the products they are to provide.
1.3.1 Temperate hardwoods
These hardwood trees are found where the cli-
mate is of a temperate nature. The temperate
regions stretch north and south from the tropical
areas of the world, into the USSR, Europe,
China and North America in the Northern
Hemisphere, and Australia, New Zealand and
South America in the Southern Hemisphere.
The United Kingdom is host to many of these
trees, but not in sufficient quantities to meet all
its needs. It must therefore rely on imports from
countries which can provide such species as oak
(Quercus spp.), sycamore (Acer spp.), ash
(Fraxinus. spp.), birch (Betula spp.), beech
(Fagus spp.), and elm (Ulmus spp.) – which is
now an endangered species due to Dutch elm
disease.
1.3.2 Tropical and subtropical hardwoods
Most tropical hardwoods come from the rain
forests of South America, Africa, and South East
Asia. Listed below are some hardwoods which
are commonly used:
African mahogany – West Africa
(Khaya spp.)
†Afrormosia (Pericopsis – West Africa
elata)
Agba – West Africa
(Gossweiterodendron
balsamiferum)
6 Timber
Table 1.1 Guide to recognising hardwood and softwood trees and their use
Hardwoods Softwoods (conifers)
Botanical grouping Angiosperms Gymnosperms
Leaf group Deciduous* and Evergreen†
evergreen
Leaf shape Broadleaf Needle leaf
or scale like
Seed Encased Naked via a cone
General usage Paper and card Paper and card
Plywood (veneers and core) Plywood (veneer and core)
Particle board Particle board
Timber – heavy structural, decorative joinery Fibre board
Timber – general structural joinery
Trade use Purpose made joinery Carpentry and joinery
Shopfitting
Note: *Within temperate regions around the world; †Not always, for example: larch trees are deciduous
‡American mahogany 
(Swietenia macrophylla) – Central & South
America
Gaboon (Aucoumea – West Africa
klaineana)
Iroko (Chlorophora – West Africa
excelsa)
Keruing (Dipterocarpus spp.) – South East Asia
Meranti (Shorea spp.) – South East Asia
Sapele – West Africa
(Entandrophragma
cylindricum)
Teak (Tectona grandis) – Burma,Thailand
Utile (Entandophragma utile) – West Africa
N.B. Many of these species may have originated
from tropical rain forest regions which the timber
industry is trying to control as conservation areas:
† � Trading restrictions
‡ � Protected species
Also, see table 1.15
1.3.3 Hardwood use
Hardwoods may be placed in one or more of the
following purpose groups:
Purpose group Use
a Decorative natural beauty – colour
and/or figured grain
b General-purpose joinery and light
structural
c Heavy structural withstanding heavy loads
1.3.4 Softwoods
Most timber used in the UK for carpentry and
joinery purposes is softwood imported from
Forest distribution (source and supply of timber) 7
Table 1.2 The family tree
FAMILY
Genera
Genus GenusGenus
SpeciesSpecies Species
BEECH FAMILY
GENUS
Castanea
Fagaceae
SPECIES
(sativa)
Sweet chestnut
Hardwood example
GENUS
Quercus
SPECIES
(robur) (rubra) (alba)
OAKS
CHESTNUT
Common names
English oak American
white oak
American red oak
Common names
GENUS
Fagus
SPECIES
(robur) (rubra) (alba)
BEECHES
Common names
European
beech
American
beech
PINE FAMILY
GENUS
Picea
Pinaceae
SPECIES
(abies) (sitchensis)
Softwood example
GENUS
Tsuga
SPECIES
(heterophylla) (canadensis)
HEMLOCKS
SPRUCES
Common names
Western
hemlock
Eastern
hemlock
Common names
GENUS
Pinus
SPECIES
(sylvestris) (contorta)
PINES
Common names
Scots pine
European
redwood
Lodge pole
pine
Norway spruce
European whitewood
Sitka spruce
North America Canada & USA
Douglas Fir
Yellow Pine
Western Hemlock
Amabilis Fir
Lodgepole Pine
Eastern Spruce
Western Red Cedar
Maple
Cherry
Hickory
Walnut
Red Oak (American)
White Oak (American)
Ash
Canadian Birch
Central America &
the Caribbean
Pitch Pine
American Mahogany
Rosewood
Lignum Vitre
Central & South
America
Parana Pine
Brazilian Mahogany
Balsa
Rosewood
Lignum Vitae
Greenheart
West Africa
African Mahogany
Iroko
Afrormosia
Sapele
Obeche
Teak
Central Europe
European Oak
Ash
Walnut
European Chestnut
Elm
United Kingdom
Scots Pine
Sitka Spruce
Whitewood
Douglas Fir
Larch
Alder
Oak (English)
Ash
Birch
Beech
Sweden & Finland
European Redwood
European Whitewood
Birch Russia
European Redwood
European Spruce (Whitewood)
Ash
Beech
Philippines & Japan
Lauan
Oak
South East Asia
Teak
Seraya
Meranti
Keruing
Romin
Jelutong
Australasia
Radiata Pine
Eucalyptus
Kauri
Silky Oak
Karri
Jarrah
Canada
USA
Florida
Cuba
Honduras
PACIFIC OCEAN
PACIFIC
OCEAN
South
America
Brazil
West
Africa Africa
Ghana
Nigeria
Australia
Indonesia
India
China
Russia
Asia
NORTH
SEA
ATLANTIC
OCEAN
ARCTIC OCEAN
Japan
Malaysia
Papua
New
Guinea
Norway
Sweden
Finland
Mexico
KEY:
Softwoods
(Conifers)
Temporate
Hardwoods
Mixed
Softwoods
(Conifers
and
Temporate
Hardwoods)
Tropical
Hardwoods
INDIAN OCEAN
EQUATOREQUATOR
Fig 1.7 Forestregions of the world
Sweden, Finland, and the USSR. The most
important of these softwoods are European red-
wood (Pinus sylvestris), which includes Baltic
redwood, and Scots pine, a native of the British
Isles. As timber, these softwoods are collectively
called simply ‘redwood’. Redwood is closely fol-
lowed in popularity by European Whitewood, a
group which includes Baltic Whitewood and
Norway spruce (Picia abies) – recognised in the
UK as the tree most commonly used at
Christmas as the Christmas tree. Commercially,
these, and sometimes silver firs are simply
referred to as ‘whitewood’.
Larger growing softwoods are found in the
pacific coast region of the USA and Canada.
These include such species as Douglas fir
(Pseudotsuga menziesii) – known also as
Columbian or Oregon pine, although technically
not a pine.Western hemlock (Tsuga heterophylla),
and Western red cedar (Thuj‘a plicata). Western
red cedar is known for its durability and its
resistance to attack by fungi. Brazil is the home
of Parana pine (Araucaria angustifolia), which
produces long lengths of virtually knot-free
timber, which is however, only suitable for
interior joinery purposes.
1.3.5 Forms of supply
Softwood is usually exported from its country of
origin as sawn timber in packages, or in bundles.
It has usually been pre-dried to about 20% m.c.
(Moisture content – see section 1.7). Packaged
timber is to a specified quality and size, bound or
bonded with straps of steel or plastics for easy
handling, and wrapped in paper or plastics sheets.
Hardwood, however, may be supplied as sawn
boards or as logs to be converted (sawn) later
by the timber importer to suit the customer’s
requirements.
1.4 Conversion into timber
Felling (the act of cutting down a living tree) is
carried out when trees are of a commercially
suitable size, having reached maturity, or for
thinning-out purposes. Once the tree has been
felled, its branches will be removed, leaving the
trunk (stem) in the form of a log.The division of
this log into timber sections is called conversion.
What, then, is the difference between wood
and timber? The word wood is often used very
loosely to describe timber, when it should be
used to describe either a collection of growing
trees or the substance that trees are made of, i.e.
the moisture-conducting cells and tissues etc.
Timber is wood in the form of squared boards or
planks etc.
Initial conversion may be carried out in the
forest whilst the log is in its green (freshly felled)
state by using heavy, yet portable machines, such
as circular saws or vertical and horizontal band-
mills (see fig. 1.14). This leads to a reduction in
transport cost, as squared sectional timber can
be transported more economically than logs.
Alternatively, the logs may be transported by
road, rail, or water to a permanently sited
sawmill. Were they are kept wet, either within a
log pond, or with water sprinklers.
1.4.1 Sawing machines
The type of sawing equipment used in a sawmill
will depend on the size and kind of logs it han-
dles. For example:
a Circular saw (fig. 1.8) – small- to medium
diameter hardwoods and softwoods. Figure
1.8 shows a rolling table log saw. The tables
are available in lengths from 3.05 m to 15.24
m, and the diameter of saw could be as large
as 1.829 m.
b Vertical frame saw or gang saw (fig. 1.9)
– small to medium-diameter softwoods. The
log is fed and held in position by fluted
rollers while being cut with a series of
reciprocating upward-and-downward
Conversion into timber 9
Fig 1.8 Circular saw
moving). saw blades. The number and
position of these blades will vary according
to the size and shape of each timber section.
Figure 1.10(a) illustrates the possible result
after having passed the log through this
machine once, whereas figure 1.10(b) shows
what could be achieved after making a
further pass.
c Vertical band-mill (fig. 1.11) – all sizes of
both hardwood and softwood. Logs are fed
by a mechanised carriage to a saw blade in
the form of an endless band, which revolves
around two large wheels (pulleys), one of
which is motorised. Figure 1.12 shows an
example of how these cuts can be taken.
d Double vertical band-saw (fig. 1.13) –
small to medium logs. It has the advantage
of making two cuts in one pass.
e Horizontal band-saw (fig. 1.14) – all sizes
of hardwood and softwood. The machine
illustrated is suitable for work at the forest
site, or in a sawmill. Conversion is achieved
by passing the whole mobile saw unit (which
travels on rails) over a stationary log, taking
a slice off at each forward pass.
The larger mills may employ a semi-computerised
system of controls to their machinery, thus help-
ing to cut down some human error and pro-
viding greater safety to the whole operation.
10 Timber
Fig 1.9 Vertical frame saw or gang saw
(a) First cut (b) Second cut
Fig 1.10 Possible cuts of a frame saw (see fig 1.13)
Fig 1.11 Vertical band-mill
1
2
3
4
5
6
7
8
Fig 1.12 Band-mill cuts
The final control and decisions, however, are usu-
ally left to the expertise of the sawyer (machine
operator).
Timber which requires further reduction in
size is cut on a resaw machine. Figure 1.15
shows a resawing operations being carried out,
one a single unit, and the other using two
machines in tandem to speed up the operation.
Importers of timber in the United Kingdom
may specialise in either hardwoods or softwoods,
or both. Their sawmills will be geared to meet
their particular needs, by re-sawing to cus-
tomers’ requirements. Hardwood specialists
usually have their own timber drying facilities.
Conversion into timber 11
Bansaw blades
Fig 1.13 Double vertical band-saw
Fig 1.14 ‘Forester-150’ horizontal band-mill –
through and through sawing
Fig 1.15 Resawing timber
1.4.2 Method of conversion
The way in which the log is cut (subdivided) will
depend on the following factors:
● type of sawing machine,
● log size (diameter or girth)
● type of wood,
● condition of the wood – structural defects
etc.(see section 1.6),
● proportion of heartwood to sapwood,
● future use – structural, decorative, or both.
Broadly speaking, the measures taken to meet
the customer’s requirements will (with the
exception of the larger mills) be the responsibil-
ity of the experienced sawyer (as mentioned ear-
lier), whose decision will determine the method
of conversion, for example:
a Through-and-through-sawn (fig. 1.16)
In this method of conversion, parallel cuts
are made down the length of the log,
producing a number of ‘quarter’ and
‘tangential’ sawn boards (figs 1.17 and
1.19). The first and last cuts leave a portion
of wood called a ‘slab’. This method of
conversion is probably the simplest and least
expensive.
NB. Cuts may be made vertically or
horizontally depending on the type of machine.
b Tangential-sawn (Plain sawn) – figure 1.17
shows that by starting with a squared log,
tangential-sawn boards are produced by
working round the log, by turning it to
produce boards, all of which (except the
centre) have their growth rings across the
boards’ width. Figure 1.18 shows alternative
methods leaving a central ‘boxed heart’
Although tangential-sawn sections are
12 Timber
Slab
Fig 1.16 Through and through sawn (producing
plain and quarter sawn timber)
Plain sawn timber - growth rings meet the face of the
board at an angle less than 45°
Fig 1.17 Tangentially sawn (producing ‘plain sawn’
or ‘flat sawn’ timber – except for heartboards)
Fig 1.18 Dividing the log to produce plain sawn
timber and a boxed heart
subject to cupping (becoming hollow across
the width) when they dry, they are the most
suitable sections for softwood beams, i.e.
floor joists, roof rafters, etc., which rely on
the position of the growth ring to give
greater strength to the beam’s depth.
c Quarter (radial) or rift-sawn (fig. 1.19) –
this method of conversion can be wasteful
and expensive, although it is necessary where
a large number of radial or near radial-sawn
boards are required. Certain hardwoods cut
in this fashion, producebeautiful figured
boards (fig. 1.21), for example, figured oak,
as a result of the rays being exposed (fig.
1.5). Quarter-sawn boards retain their shape
better than tangential-sawn boards and tend
to shrink less, making them well suited to
good-class joinery work and quality flooring.
The resulting timber, with the exception of that
which surrounds the ‘heart wood’ shown in table
1.3 will either be:
● Tangentially (plain) sawn.
● Quarter sawn or Rift sawn.
Conversion into timber 13
Quartered log
WasteQuarter/rift - sawn methods
Radial quarter sawn
Acceptable quarter sawn
not less than 45˚
Close-up
Fig 1.19 Quarter (radial) or rift sawn timber
Table 1.3 Comparison between ‘plain’ and ‘quarter’
sawn timber
Advantages Disadvantages
Plain sawn
Economical conversion Tends to ‘cup’ (distort) on
Ideal section for drying due to shrinkage –
softwood beams (‘cupping’ is its natural
Can produce a decorative
pattern of shrinkage)
pattern (flower or flame
figure) on the tangential
face of the timber with
distinct growth rings –
see Figures 1.21 and 1.86c
Quarter sawn
Retains it shape better Expensive form of
during drying conversion
Shrinkage across its width Conversion methods can
half of that of plain sawn be wasteful
timber
Ideal selection for flooring
with good surface wearing
properties
Produces a decorative
radial face (e.g. Silver
Figure) on hardwoods
with broad ray tissue,
see figures 1.21 and
1.86 b and c
1.4.3 Conversion geometry (Fig. 1.20)
Knowing that a log’s cross-section is generally
just about circular, the above-mentioned saw
cuts and sections could be related to a circle and
its geometry. For example, timber sawn near to a
‘radius’ line will be radial-sawn. quartered logs
(divided by cutting into four quarters) or quad-
rants. Similarly, any cut made as a tangent to a
growth ring would be called tangentially sawn.
The ‘chords’ are straight lines, which start and
finish at the circumference; therefore a series of
chords can be related to a log that has been sawn
‘through-and-through’, ‘plain sawn’ or ‘flat
sawn’. It should be noted that the chord line is
also used when cuts are made tangential to a
growth ring, and when the log is cut in half.
1.4.4 Decorative boards
Figure 1.21 gives two examples of how wood
can be cut to produce timber with an attractive
face.
Quarter sawn hardwoods with broad rays can
produce nicely figured boards. For example,
quarter sawn European Oak is well known for
its ‘Silver figure’ when sawn in this way.
Tangentially sawn softwoods with distinct
growth rings can produce a flame like pattern on
their surface – known as ‘Flame figuring’.
Further examples can be see in figure 1.86.
1.5 Size and selection of sawn
timber)
Sawn timber is available in a variety of cross-
sectional sizes and lengths to meet the different
needs of the construction and building industry.
By adopting standard sizes, we can reduce the
time spent on further conversion, subsequent
wastage, and the inevitable build-up of short ends
or off-cuts (off-cuts usually refers to waste pieces
of sheet materials), thereby making it possible to
plan jobs more efficiently and economically.
1.5.1 Softwoods
Depending on whether the suppliers are from
North America or Europe, stated cross- section
sizes can vary. Canadian mills, unlike European
mills, may not make any allowance in their sizes
for any shrinkage when their timber is dried.
Timber shrinks very little in its length, so
allowance provisions are not necessary.
Table 1.4(a) shows the cross sectional sizes of
sawn softwood normally available in the U.K.
and table 1.4(b) their cut lengths.
1.5.2 Hardwoods
As shown in figure 1.22 different profiles are
available to suit the end user. Dimentioned sawn
stock sizes as shown in table 1.5 may be avail-
able, but this will depend on species and local
availability.
14 Timber
Segment (slab)
Plain sawn
Chords
Quarter
Sawn
QuadrantR
ad
iu
s
Diameter
Circumference
ArkL
og
Ta
ng
en
t
Ta
ng
en
t
90
�
Norm
al
Fig 1.20 Conversion geometry
Rays
Exposed
earlywood
Exposed
latewood
Exposed broad
‘ray’ tissue
Plain sawn Douglas fir
or European Redwood
Quarter sawn Oak
or Beech
Fig 1.21 Decorative boards (other examples are
shown in fig. 1.86)
Figures 1.23 to 1.27 show defects that may be
evident before, and/or during conversion. Most
of these defects have little, if any, detrimental
effect on the tree, but they can degrade the tim-
ber cut from it, i.e. lower its market value.
1.6.1 Reaction wood (fig 1.23)
This defect is the result of any tree which has had
to grow with a natural leaning posture, this may
be as a result of having to resist strong prevailing
winds, or having to established itself on sloping
ground. These trees resist any pressure existed
upon them by attempting to grow vertically with
Structural defects (natural defects) 15
Table 1.4 Sawn sizes of softwood timber
(a) Customary target sizes of sawn softwood
Thickness Width (mm)
(mm) 75 100 115 125 138 150 175 200 225 250 275 300
16 � � � �
19 � � � �
22 � � � �
25 � � � � � � � � � �
32 � � � � � � � � � � �
38 � � � � � � � � � � � �
47 � � � � � � � � �
50 � � � � � � � � �
63 � � � � � �
75 � � � � � � � � �
100 � � � � � � �
150 � � �
250 �
300 �
Note: Certain sizes may not be obtainable in the customary range of species and grades which are generally available.
Permitted deviation of cross-sectional sizes at 20% moisture content.
● for thickness and widths �100 mm [�3
�1
] mm;
● for thickness and widths �100 mm [�4
�2
] mm.
Target size of 20% moisture content.
(b) Customary lengths of sawn softwood
1.80 2.10 3.00 4.20 5.10 6.00 7.20
2.40 3.30 4.50 5.40 6.30
2.70 3.60 4.80 5.70 6.60
3.90 6.90
Note: Lengths of 5.70 m and over may not be
readily available without finger jointing
See Table 1.5
5.7
00
–m
ay 
go 
up
 to
 7.2
00
1.8
00
30
0 m
m
Inc
rem
ent
s
(st
age
s)
Dimensioned
stock
Random width -
one straight edge
Random width -
waney edged 
Fig 1.22 Profiles of hardwood sections
1.6 Structural defects (natural
defects)
(c)
added supportive wood growth to their stem.
This extra growth will be formed in such a way
that the stem will take on an eccentric appear-
ance around the stem, which, with softwood is
on the side of the tree that is being subjected to
compressive forces – this wood is known as a
compression wood. Hardwoods on the other
hand, produce extra wood on the side likely to
be stretched, since this is the side in tension.
This wood is known as tension wood. In both
these cases the wood is unsuitable as timber
since it would be unstable went dried and par-
ticularly hazardous when processed. Collectively,
both compression and tension wood are known
as reaction wood.
1.6.2 Heart shake (Star shake – Fig. 1.24(a))
Shake (parting of wood fibres along the grain)
within the heart (area around the pith) of the
tree caused by uneven stresses, which increase as
the wood dries. A star shake is collection of
shakes radiating from the heart.
16 Timber
Lean due to prevailing wind
Lean due to steep natural propogation
Compression
side
(a) Compression woods
(Softwoods)
(b) Tension woods
(Hardwoods)
Tension
side
Section A-ASection B-B
A A
B B
Fig 1.23 Reaction wood
(e)
(f)
(g)
(b)
(a)
(d)
(c)
Fig 1.24 Structural (natural) defects
Table 1.5 Basic guide sizes of sawn Hardwood
Thickness Width (mm)
(mm) 50 63 75 100 125 150 175 200 225 250 300
19 � � � � �
25 � � � � � � � � � � �
32 � � � � � � � � � �
38 � � � � � � � �
50 � � � � � � � �
63 � � � � � �
75 � � � � � �
100 � � � � � �
Note: Designers and users should check the availability of specified sizes in any particular species
1.6.3 Ring shake (Cup shake – Fig. 1.24(b))
A shake which follows the path of a growth ring.
Figure 1.24 (c) shows the effect it can have on a
length of timber.
1.6.4 Natural compression failure (upset –
fig. 1.24(e))
Fracturing of the fibres; thought to be caused by
sudden shock at the time of felling or by the tree
becoming over-stressed(during growth) – possi-
bly due to strong winds etc.
NB. Other names for this defect include
‘thunder shake’ or ‘lightning shake’.
1.6.5 Rate of growth fig. 1.24(d))
The number of growth rings per 25 min, can
with softwoods determine the strength of the
timber.
1.6.6 Wane (Waney-edge – fig. 1.24(f))
The edge of a piece of timber that has retained
part of the tree’s rounded surface, possibly
including some bark.
1.6.7 Encased bark (fig. 1.24(g))
Bark may appear inset into the face or the edge
of a piece of timber.
1.6.8 Sloping grain (fig. 1.25)
The grain (direction of the wood fibres), slopes
sharply in a way that can make load-bearing tim-
bers unsafe, e.g. beams and joists.
Figure 1.25a shows possible source. Figure
1.25b a method of testing for sloping grain.
Figure 1.25c how sloping grain could be respon-
sible for pre-mature fracturing of a beam.
1.6.9 Knots
As shown in figure 1.26 where the tree’s branches
join the stem they become an integral part of it.
The lower branches are often trimmed off by for-
est management during early growth, this encour-
ages ‘clear wood’ to grow over the knot as the tree
develops.
Figure 1.27 shows how knots may appear in
the sawn timber. The size, type, location, and
number of knots, are controlling factors when
the timber is graded for use.
Some of the terms used to describe knots are:
● Dead knots – If a branch is severely
damaged, that part adjoining the stem will
die and may eventually become enclosed as
the tree develops – not being revealed until
Structural defects (natural defects) 17
a) Timber sawn
from bent log
b) Testing for
slope of grain by
pulling a swivel handled
scribe along the grain
Kg
c) Premature Fracture
Fig 1.25 Sloping grain
Knots radiating 
from 'Pith'
A
B
C
Face splay or
spike knot
Edge knot Edge splay or
spike knot
Arris
Knot
Face
knot
Loose 'dead'
margin 
knot
Branch
Fig 1.26 Knots in relation to branches and stem
Location
conversion into timber. Note: these knots are
often loose, making them a potential hazard
whenever machining operations are carried out.
● Knot size – Larger the knot greater the
strength reduction of the timber.
● Knot location – Knots nearer the edges
(margins) of the beam are generally going to
reduce the strength properties of timber,
rather than those nearer the centre.
● Number of Knots – Generally the greater
the distance between the knots the better.
● Knot types – Knots appearing on the
surfaces of timber take many forms the
names reflect their position, for example:
● Face knots
● Margin knots
● Edge knots
● Arris knots
● Splay or spike knots
1.6.10 Resin (Pitch) pocket
An a opening, following the saucer shape of a
growth ring containing an accumulation of resin.
Apparent in many softwoods, mainly in spruces
– it may appear as a resinous streak on the sur-
face of timber. In warm weather sticky resin may
run down vertical members. When the resin
dries it takes on a resinous granular form which
can be scraped away.
1.7 Drying timber
Timber derived from freshly felled wood is said
to be green, meaning that the cell cavities contain
free water and the wall fibres are saturated with
bound water (fig. 1.33), making the wood heavy,
structurally weak, susceptible to attack by insects
and/or fungi, also unworkable. Timber in this
condition is therefore always unsuitable for use.
The amount of moisture the wood contains as a
percentage of the oven-dry weight, is known as
the moisture content (m. c.), and the process of
reducing the m.c is termed drying.
The main object of drying timber is therefore
to:
● reduce its weight,
● increase its strength properties,
● increase its resistance to fungal and attack by
some insects,
18 Timber
Whorl
(circular set)
of branches
knots
distributed
knot
cluster
Fig 1.27 Knot condition, size and distribution
Depth
D
0.25( � )D₁
0.25( � )D₁
Margin
areas
Large knot
Pin
knots
Dead knot
Live/sound
or tight-knot
Loose
dead knot
● provide stability with regards to moisture
movement,
● increase workability for machine and hand
tools
● enable wood preservative treatments to be
applied, (with the exception of those applied
by diffusion – section 3.4.2)
● enable fire retardant treatments to be applied
● enable surface finishes to be applied
● enable adhesives to be applied,
● reduce the corrosive properties of some
woods,
● reduce heat conductivity thereby increase
thermal insulation properties,
and produce timber with a level of moisture con-
tent acceptable for its end use. Examples are
given in figure 1.28 and table 1.6.
The drying process, (sometimes called ‘sea-
soning’), is usually carried out by one of three
methods:
a Air-drying (natural drying),
b Kiln-drying (artificial drying),
c Air-drying followed by kiln-drying.
All three methods aim at producing timber that
will remain stable in both size and shape – the
overriding factor being the final moisture con-
tent, which ultimately controls the use of the
timber.
Although outside the scope of this book other
drying methods include:
● forced-air drying,
● climate chambers,
● dehumidifiers,
● vacuum drying,
Because the object of drying timber is to remove
water from the cells (fig. 1.33), moisture content
is considered first.
1.7.1 Moisture content (m.c)
As already expressed, the moisture content of
wood is the measured amount of moisture within
a sample of wood expressed as a percentage of
its dry weight. If the weight of water present
exceeds that of dry wood, then moisture con-
tents of over 100% will be obtained.
There are several methods of determining
moisture content values, but we will only be con-
sidering the following two methods:
● the traditional oven-drying method, and
● using modern electrical moisture meters and
probes,
a Oven-drying method (fig. 1.29) – a small
sample cut from the timber which is to be
dried (see fig. 1.43) is weighed to determine
its ‘initial’ or ‘wet’ weight. It is then put into
an oven with a temperature of 103�C 	 2�C
until no further weight loss is recorded, its
weight at this stage being known as its ‘final’
or ‘dry’ weight.
Once the ‘wet’ and ‘dry’ weights of the
sample are known, its original moisture
content can be determined by using the
following formulae:
Moisture content % �
Initial (wet weight (A)) � final (dry weight (B))
� 100
Final or dry weight (B)
Or
Initial (wet weight (A))
MC % �1 � 100
Final or dry weight (B)
For example, if a sample has a wet weight of
25.24 g (A) and a dry weight of 19.12 g (B), then:
A � B
MC % � � 100
B
25.24 g � 19.12 g
� 100 � 32%
19.12 g
Drying timber 19
N.B. Wood with a 20% + M.C. is liable to attack by fungi
15 to 20%
8%
12%17% 12% 18%
Fig 1.28 Moisture content of wood products in
various situations
Or
A
MC % � �1 � 100[( B ) ]
25.24 g
MC % � �1 � 100 � 32%[( 19.12 g ) ]
b Electrical moisture meters (fig. 1.30)
These are battery-operated instruments,
which usually work by relating the electrical
resistance of timber to the moisture it contains.
Moisture content is measured by pushing or
driving (hammer-type) two electrodes into the
timber. The electrical resistance offered by the
timber is converted to a moisture content which
can be read off a calibrated digital scale of the
meter, the lower the resistance, the greater the
moisture content, since wet timber is a better
conductor of electricity than dry.
Meters generally will only cope with accuracy,
for timber with moisture content between 6 and
28%. Above this point (the fibre-saturation
point) there will be little or no change in electri-
cal resistance. With the exception of the small
hand held models (fig 1.31) useful for making
20 Timber
Table 1.6 Moisture content of timber in relation to its end use
10
5
0
% Shrinkage in relation to
section and moisture content
(N.B. Guide only as there can be
great variations between species) 
MC %MC %
Radial shrinkage
(See section 1.7.3)
Internal joinery–occasionally heated buildings
A
rt
if
ic
ia
l 
d
ry
in
g
 (
k
iln
s
) 
n
e
c
e
s
sa
ry
A
ir
 d
ry
in
g
 c
a
n
 b
e
 u
s
e
d
Internal joinery–intermittently heated buildings
Internal joinery–continuously heated buildings
Internal joinery close to heat source
Oven dry
1
0
2
3
4
7
6
8
9
12
11
13
14
17
16
18
19
22
21
23
27
26
28
25
20
15
10
5
24
Internal joinery–continuously highly heated
buildings e.g.hospitals, offices, etc
External joinery and structural timbers
Above this line dry rot spores may germinate
Carcassing timber (to average 20% MC)
Shrinkage begins here
Pressure preservative treatment with creosote
or CCA, a flame retardent treatment
Tangential shrinkage
(see Section 1.7.3)
2 4 6 8 10
25
20
15
spot checks on site. Moisture meters are in two
parts (fig 1.32):
● The meter itself with both a numerical scale
and pointer, or digital readout – provision
will be made for adjustment to suit different
wood species – this part will also have
provision for housing the batteries. It may also,
like the one shown in figure 1.32 have integral
pins to allow surface readings to be taken.
● Spiked electrodes (probes) set into an
insulated hand-piece, with provision for
attaching it to the meter via a detachable
cable.
Moisture meter systems are more than just a
useful aid for making spot checks – in fact in the
Drying timber 21
Dried sample
* (See Fig. 1.43 -'Cutting
oven samples') 
B
Fig 1.29 Method of determining moisture content
by oven drying a small sample of timber (also see fig 1.43)
Oven drying
% M.C. = x100
A – B
B
Wet sample *
A
Small current
from battery
INPUT OUTPUT
Minimal
current flow -
good resistance
offered
INPUT OUTPUT
Small 'wet'
timber sample
Small 'dry'
timber sample
Current flow -
little resistance
offered
Conductivity increases with any increase in 
moisture content
Analogue Digital
Hammer
action
Push-in-type electrodes
(thin timber sections) Hammer-in-type
electrodes
(thicker timber sections)
Needles
(electrodes)
Wet zone
(high moisture content)
Dry zone
(lower moisture 
content)
Approximate range 
of recordable moisture
content 6-30%
Fig 1.30 Battery operated moisture meter
practical sense, when used in conjunction with
the timber drying procedures of air and kiln dry-
ing, they can be better than the oven-drying
method.Whenever a moisture meter is used it is
important that:
a probes can reach the part of the timber
whose moisture content is needed (depends
on the sectional size of the timber and the
type of instrument);
b allowance must be made for the timber
species – timber density can affect the
meter’s reading;
c the temperature of the timber is known –
meter readings can vary with temperature;
d certain chemicals are not present in the
timber, for example, wood preservatives or
flame-retardant solutions.
Tests on moisture content may be necessary
when sorting large batches of timber, or check-
ing the condition of assembled or fixed carpen-
try and joinery, particularly, if a fungal attack is
in evidence or suspected – in which case a mois-
ture meter would be invaluable.
1.7.2 Moisture removal
Before considering the two main drying tech-
niques, let us try to understand how this loss of
moisture may effect the resulting timber. Figure
1.33 illustrates how moisture is lost naturally,
and the effect it can have on a timber section if
moisture is then reintroduced.
We already know that green timber contains
a great amount of water. This water is con-
tained within the cell cavities – we call this free
water, because it is free to move around from
cell to cell. The water contained within the cell
walls is fixed (chemically bound to them), and
is therefore known as bound water or bound
moisture.
As you will know the air we breathe contains
varying amounts of moisture: the amount will
depend on how much is suspended in the air as
vapour at that point in time, which in turn will
depend on the surrounding air temperature. As
the air temperature increases, so does its capac-
ity to absorb more moisture as vapour, until the
air becomes saturated, at which point we are
very aware of how humid it has become. It is
therefore this relationship between air tempera-
ture, and the amount of moisture the air can
hold that we call relative humidity.
If the air, surrounding the timber has a vacant
capacity for moisture, it will take up any spare
moisture from the wood until, eventually, the
moisture capacity of the air is in balance, or
equilibrium, with that of the timber.When stable
22 Timber
Fig 1.31 Hand held ‘mini’ moisture metre by
‘protimeter’ (with kind permission from Protimeter Ltd)
Fig 1.32 Protimeter diagnostic timber master – two
part moisture meter (with kind permission from
Protimeter Ltd)
conditions are reached we can say an equi-
librium moisture content (EMC) has been
achieved. This process will of course act in
reverse, because wood is a hygroscopic mater-
ial, which means that it has the means, provided
the conditions (those mentioned above), are
suitable, to pick up from and shed moisture to
its surrounding environment.
Any free water will leave first, via tiny perfora-
tions within the cell walls. As the outer cells of
the timber start to dry, they will be replenished
by the contents of the inner cells, and so on,
until only the cell walls remain saturated.The
timber will start to shrink at this important stage
of drying, known as the fibre saturation point
(FSP) when about 25% to 30% m.c. (table 1.6)
will be reached.
Beyond fibre saturation point (FSP), drying
out bound water can be very lengthy process if
left to take place naturally. To speed up the
process, artificial drying techniques will need to
be employed.
It is worth pointing out at this stage, that it is
possible for timber in a changeable environment
to remain stable if moisture absorption can be
prevented. This may be achieved by one of two
methods:
1 Completely sealing all its exposed surfaces,
2 Using a micro-pore sealer that prevents
direct entry of water from outside but
allows trapped moisture to escape.
All timber must of course be suitably dried
before any such treatments are carried out.
Drying timber 23
HIGH MOISTURE (M)
CONTENT
EQUILIBRIUM
(depends on environment)
INCREASED MOISTURE CONTENT
m
M
E
E
E E m
m
m
mm
Shrinkage ExpansionNo shrinkage
Cell
Cell cavity-free water
GREEN DRYING DRY (seasoned)
Moisture absorption
M.C. retained by sealing pores
with paint, varnish etc.M/m = moisture E = evaporation
Cell wall
 (Bound moisture)
Fibre saturation
Fig 1.33 Basic principles of moisture movement
1.7.3 Wood shrinkage
Whether natural or artificial means are used to
reduce the moisture content of timber, it will
inevitably shrink. The amount of shrinkage will
depend on the reduction below its FSP.
Probably the most important factor, is the
relationship between the differing amounts of
shrinkage, compared with the timbers length
(longitudinally), and its cross-section (transverse
section), whether it is plain saw (tangentially) or
quarter saw (radially). And how, as shown in fig-
ure 1.34 we can view different proportions of
shrinkage, for example:
a tangentially – responsible for the greatest
amount of shrinkage
b radially – shrinkage of about half that of
tangential shrinkage
c longitudinally – hardly any shrinkage.
We call varying amounts of shrinkage differen-
tial shrinkage.
Figure 1.35 shows how shrinkage movement
takes place in relation to the direction of the
wood cells situated across the end grain. As a
result of this movement, we can expect some sec-
tions of timber to distort in some way as mois-
ture is removed from the cells to below fibre sat-
uration point. The resulting shapes of distorted
timber sections will depend on where the timber
was cut out of the log during its conversion into
timber. Figure 1.36 should give some idea as to
how certain sections of timber may end up after
being dry – reference should be made to section
1.7.8 which itemises various drying defects.
1.7.4 Air drying (natural drying)
Oftencarried out in open-sided sheds, where the
timber is exposed to the combined action of
circulating air and temperature, which lifts and
drives away unwanted moisture by a process of
evaporation (similar to the drying of clothes on a
washing line). A suitable reduction in m. c. can
take many months, depending on:
24 Timber
a tangentially - responsible for the greatest amount of 
shrinkage
b radially - shrinkage of about half that of tangential
shrinkage
c longitudinally - hardly any shrinkage
'C'
(c) Length (longitudinal)
minimal shrinkage (least amount)
Plain saw
'A'
'B'
Quarter
 sawn
'C' 'C'
(a) Tangent
Tangentially - the greatest
amount of shrinkage
(b) Radial
Radially shrinkage about 
half the amount of tangential
Fig 1.34 Proportions of wood shrinkage
Radial cell shrinkage reduced to about half
of tangential cell movement - this 
restriction is due in part 
to lack of 
movement
of ray cells radially
Greatest shrinkage
accross the
cells tangentially
Ray cells
(little or no
movement
radially)Axial cells
(little or no
Movement)
Fig 1.35 Shrinkage movement in relation to
direction of wood cells (exaggerated view of end grain)
a the drying environment and amount of
exposure,
b the type of wood (hardwood or softwood),
c the particular species,
d the timber thickness.
The final m. c. obtained can be as low as 16 %
to 17 % in summer months and as high as 20 %
or more during winter. It would therefore be fair
to say that this method of drying timber is very
unreliable.
A typical arrangement for air-drying is shown
in figure 1.37, where the features numbered are
of prime importance if satisfactory results are to
be achieved. They are as follows:
1 Timber stacks (piles of sawn timber), must
always be raised off the floor, thus avoiding
rising damp from the ground. Stacked
correctly (fig. 1.39). Concrete, gravel, or
ash will provide a suitable site covering.
2 The area surrounding the shed must be
kept free from ground vegetation, to avoid
conduction of moisture from the ground.
3 Free circulation of air must be maintained
throughout the stack – the size and position
of ‘sticks’ will depend on the type, species,
and section of timber being dried.
4 The roof covering must be sound, to protect
the stacks from adverse weather conditions.
The success of air-drying will depend on the fol-
lowing factors:
a weather protection,
b site conditions,
c stacking as shown in figure 1.39,
d atmospheric conditions.
a Weather protection
Except when drying certain hardwoods which
can be dried as an open-piled ‘boule’ (the log
being sawn through-and-through and then
reassembled into its original form – see
‘Stacking’), a roof is employed to protect the
stack from direct rain or snow and extremes in
temperature. Its shape is unimportant, but cor-
rugated steel should be avoided in hot climates
because of its good heat-conducting properties
that would accelerate the drying process. Roof
coverings containing iron are liable to rust and
should not be used where species of a high
tannin content (such as Oak, Sweet chestnut,
Afrormosia, Western red cedar, etc.) are being
Drying timber 25
Minimal shrinkage distortion
Diamonding
Cupping
Fig 1.36 Shrinkage – its possible effect on timber
IMPORTANT ELEMENTS:
1. Risen off the ground - no rising damp.
2. Clear of ground vegetation.
3. Free circulation of air.
4. Protection from the weather.
Sticks
(stickers) at
0.600 to 1.200
4
3
2
1
Fig 1.37 Air drying shelter and stack build-up
dried, otherwise iron-staining is possible where
roof water has dripped on to the stack.
Shed sides may be open (fig. 1.37) or slatted.
Adjustable slats enable the airflow to be regu-
lated to give greater control over the drying
process. End protection can also be provided by
this method – unprotected board ends are liable
to split as a result of the ends drying out before
the bulk of the timber, hardwoods like oak and
beech are particularly prone to this problem.
Other methods used to resist this particular sea-
soning defect are shown in figure 1.38, namely:
● treating the end grain with a moisture-proof
sealer – for example, bituminous paint or
wax emulsion, etc.;
● nailing laths over the end grain – thick laths
should be nailed only in the middle of the
board, to allow movement to take place;
● hanging a drape over the end of the boule or
stack.
b Site conditions
As previously stated the whole site should be
well drained, kept free from vegetation by blind-
ing it with a covering of ash or concrete, and kept
tidy.
If fungal or insect attack is to be discouraged,
‘short ends’ and spent piling sticks should not be
left lying around.
Sheds should be sited with enough room left
for loading, unloading, carrying out routine
checks, and other operations.
c Stacking the timber (fig 1.39)
The length of the stack will be unlimited
(depending on the timber lengths), but its height
must be predetermined to ensure stability, and
the stack must be built to withstand wind. The
width should not exceed 2 metres, otherwise
crossed airflow may well be restricted to only
one part of the stack, however, adjacent stacks
can be as close to each other as 300 mm.
d Piling sticks (stickers)
Piling sticks (stickers) should never be made
from hardwood, or they could leave dark marks
across the boards (fig 1.52). Their size and dis-
tance apart will vary, according to board thick-
ness, drying rate, and species. They must always
be positioned vertically one above the other,
otherwise boards may ‘bow’ as shown in figure.
1.39(a). Stacks with boards of random length
may require an extra short stick as shown in fig-
ure 1.39 (b).
26 Timber
Roll-up drape (tarpaulin ect.)
Moisture-proof
 coating-bituminous
 paint or wax 
 emulsion
Allows wood
slab to shrink
Thin lath -
Alowed to buckle
Thin lath -
three nails
Thick lath -
centre-nails
Fig 1.38 End grain protection of timber or boule
will help prevent end splitting
Softwood sticks 25mm x 13mm to 25mm x 25mm at
intervals of 0.600 to 1.200 centres -
depending on board thickness and drying rate
Short sticks
Not in-line
In-line
No support
Sticks in-line
(a)
(b)
Fig 1.39 Build-up of stack
Figure 1.40 shows how boules are piled in log
form. Certain hardwoods are often dried in this
way, to ensure that the dried boards will match
one another in colour and grain figure.
e Atmospheric conditions in general
It is impracticable to generalise on an ideal dry-
ing environment when atmospheric conditions
can vary so much between seasons and coun-
tries. It is, however, important that whatever
means are used to regulate the drying rate of
timber, should be directed at achieving unifor-
mity throughout the whole stack – otherwise, the
timber could become distorted or suffer other
defects due to uneven shrinkage (see ‘Drying
defects’), section 1.7.8.
1.7.5 Kiln drying (artificial drying)
These kilns are generally large closeable cham-
bers into which stacks of green timber are
manoeuvred via a system of trolleys to undergo
a controlled method of drying. Kilns of this
nature dramatically reduce the drying time
compared to air drying methods, as they take a
matter the days instead of months. They vary in
their construction, size and function. There are
those where the stacks of timber remain static
(stationary) until required moisture content level
is reached; these are known as compartment kilns.
Then, there is a method were timber is moved in
stage through a tunnel dryer, known as a progres-
sive kiln. Both types of kiln will require means of
providing controlled:
● heat,
● ventilation,
● humidification,
● air circulation.
Heat is often provided via steam or hot water
pipes. The fuel used to fire the boiler may be of
wood waste, oil, gas, or coal.
Ventilation is achieved by adjustable openings
strategically positioned in the kiln wall or roof.
Alternatively, a dehumidifier can be used to
extract unwanted moisture and channel it out-
side the kiln in the form of water – thus con-
serving heat and reducingfuel costs.
When the amount of moisture leaving the
wood is insufficient to keep the humidity to the
required level, jets of steam or water droplets
may be introduced into the chamber.
Air circulation is promoted by a single large fan
or a series of smaller fans, located either above or
to the side of the stack, depending on the kiln
type.
All the above must be controlled in such a way
that the whole process can be programmed to
suit the species, thickness, and condition of the
wood. Prescribed kiln schedules are available to
take the wood through the various stages of
moisture content (say from ‘green’ to 15% m.c.).
These are listed in descending order for the
appropriate kiln temperature and relative
humidity needed at each stage of drying.
The temperature and the amount of water
vapour in the air entering the stack are measured
with a kiln hygrometer, to assess the relative
humidity of the air, which will determine the rate
at which the wood dries.
Relative humidity at a particular temperature
is expressed as a percentage (fully saturated air
having a value of 100% RH). Less water vapour
at the same temperature means a lower relative
humidity; therefore by lowering the relative
Drying timber 27
Raised off ground
Sticks 13mm to 19mm
thick - vertically in-line
Fig 1.40 Hardwood boules – piled in log form
humidity, drying potential is increased. It must
also be remembered that the higher the air tem-
perature, the greater its vapour-holding capacity.
Very broadly speaking, it can be said that kiln
drying involves three stages:
i heating up the wood without it drying –
low heat, high humidity;
ii starting and continuing drying – increased
heat, less humidity;
iii final stages of drying – high heat, slight
humidity.
Kiln samples
However, kiln adjustments required to satisfy the
schedules cannot be made until the correct
moisture content of the stack as a whole is
known. This may mean sample testing. Sample
testing may involve the use of modern battery
operated moisture meters (described earlier) or
a sample weighing method where moisture con-
tent can be calculated by a boards loss in weight.
Figure 1.41 shows how timber is piled and
provision is made in a stack for the easy removal
of board samples. Figure 1.42 shows a sample
board being removed from the stack – notice
also the control panel outside the kiln with its
relative humidity recorder.
The weighing method involves cutting out
oven samples from each sample board as shown
in figure 1.43 then finding its moisture content
by the oven dry method previously described
(section 1.7.1(a)). The kiln sample is then
weighed to obtain its wet weight; its dry weight
can then be calculated by using a simple calcu-
lation. Future checks are then made by re-
weighing the kiln sample, and cross-referencing
the results with a drying table chart.
However, by using modern control equipment
that is fully automatic the moisture content of
timber within the kiln can be continually moni-
tored, enabling the equilibrium moisture content
(EMC) to be controlled according to the wood
species and sectional size of the timber.
28 Timber
Sticks vertically in-line
Boards edge to edge Sticks cut-away
Double stick
Fig 1.41 Two examples of how provision can be
made for easy removal of kiln samples
Fig 1.42 Inspection and removal of a kiln sample
(permission Wells Ltd.)
Sample board
(kiln sample)
250mm
Oven sample
15mm
Fig 1.43 Cutting an oven sample to determine the
moisture content of the sample board
1.7.6 Compartment kilns (dryers)
These are sealable drying chambers (compart-
ments) which house batches of timber, loaded on
trolleys, until their drying schedule is complete.
Figure 1.44 shows how these kilns can be
arranged – separately with single, double, or triple
tracks, or joined together in a row (battery).
Kilns may be sited outside, like that shown in
figure 1.45 and figure 1.46, or undercover like
the battery of dryers shown in figure 1.47, which
receive heat from a central boiler plant.
Figure 1.48 shows how timber piles are
stacked in a double-track kiln with a central unit
containing the large fan, heaters, humidifiers,
ventilators, and controls. Figure 1.49 has a simi-
lar unit to one side to accommodate three tracks.
Figure 1.50 shows a compartment dryer with a
overhead circulation unit.
1.7.7 Progressive kilns (continuous dryers)
Green timber enters the kiln at one end, and
after a period of time which can be as short as
three to five days, depending on the species and
the cross-section – emerges from the exit at the
opposite end in a much drier state. The whole
process enables timber to be dried by continu-
ous means.
Figure 1.51 shows how batches of timber are
lined up on trolleys on tracks outside the dryer,
Drying timber 29
Single track
Double track
Triple
track
Track Track
Battery of
single-track
dryers
1
2
3
4
5
6
Ventilation
Track to
sealable chamber
Heating pipes
and humidifier
Fan
ciculates
air
Fig 1.44 Types of compartment dryers (for the sake
of clarity some doors are not shown)
Fig 1.45 External sited compartment dryers (with
kind permission of ‘Kiln Services Ltd’)
Fig 1.46 Externally sited battery of compartment
dryers – one being loaded with timber (with kind
promise of ‘Wells Ltd’)
ready to follow those already inside. On entry,
each batch will go through a series of stationary
drying stages, which start cool and humid but
end with the last stage being warm and dry.
When a batch leaves the dryer, a new batch will
enter from the other end to take its place.
For this method of drying to be cost effective
it requires a continuous run of timber of similar
species with common drying characteristics and
sectional size like those you might find in large
mills specialising in drying softwoods.
1.7.8 Drying defects
Successful drying depends on how drying
preparations are made and on how the whole
operation is carried out.
Green timber is usually in a pliable state –
after drying it stiffens and sets. For example, a
green twig will bend easily but, if held in that
position until dry, it will set and remain partially
bent. Therefore, if green timber is allowed to
become distorted, either by incorrect piling (fig
30 Timber
Vents
Trolley TrackFan
and drying
unit
Fig 1.48 Cross section through a ‘Wells’ double
track high-stacking prefabricated timber dryer
Vents
Trolley
Track
Fan
and drying
unit
Fig 1.49 Cross section through a ‘Wells’ triple track
high-stacking prefabricated timber dryer
Fig 1.50 Cutaway view of a double stack rail entry
kiln (kindly supplied by ‘Kiln Services Ltd’)
Fig 1.47 Undercover battery of compartment dryers
(with kind promise of ‘Wells Ltd’)
1.39) or due to unbalanced shrinkage during its
drying, then permanent degrading could result.
By using table 1.7, and the accompanying
illustrations featured in figures 1.52 to 1.62, you
should be able to recognise each of the listed
defects. These defects have been grouped as:
● Stains
● Distortions
● Checking & splitting
● Case-hardening
it can be seen that most of these degrading
defects can be attributed to both the unevenness
and the speed at which moisture is removed
from the wood.
It is important to remember at this stage how
moisture is lost from the wood as it dries. As
shown in figure 1.63 on drying, the outer surface
of the wood dries first, and as the moisture is lost
through evaporation, all things being equal it is
replaced with that contained within the wood. If
this flow is restricted due to any imbalance,
internal stresses within the wood will be created
resulting in many of the defects listed.
1.8 Grading timber
Like other natural materials with inherent varia-
tions, timber is required to meet certain stan-
dards so that it can be classified suitable for a
particular end use. Two of the main issues to be
considered here are the timbers appearance and
its strength qualities. For the purpose of this
chapter we shall call the grading rules set down
for appearance as ‘commercial grading’, and